According to trend toward use of high frequencies and high-speed digitization in data communication devices, it has been important recently to perform highly accurate circuit simulation in a time domain of electronic circuits including passive components.
Regarding the electronic devices, it is very difficult to estimate electric characteristics of a complicated electronic circuit when the circuit is being designed. For manufacturing the circuit, a lot of trial is repeated in which an actual prototype assembly of the circuit is made and measured in its electric characteristic, and the circuit is designed over again if the assembly does not exhibits desired electric characteristic.
Circuit simulations for estimating electric characteristics of the electronic circuit are performed with a circuit simulator consisting of a computer and software in order to reduce the trial. As the software, for example, a Simulation Program with Integrated Circuit Emphasis (SPICE) developed by the University of California is known.
A circuit simulation requires an equivalent circuit model, which specifies electric characteristics of semiconductor devices, such as transistors, FETs, and diodes, and passive components, such as resistors, capacitors, and inductors. It is important to establish a highly accurate equivalent circuit model of the circuit components since accurateness of the circuit simulation depends greatly upon accuracy of the equivalent circuit model.
For a capacitor, one of the passive components, equivalent circuit models that use comparatively small number of circuit components have been provided. The model includes a three-element model in which first capacitor C1, first resistor R1, and first inductor L1 are connected in series as shown in FIG. 6A, and a five-element model in which a series connection of first capacitor C1 and first resistor R1 and a series connection of second capacitor C2 and second resistor R2 are connected in parallel, and first inductor L1 is connected in series to the parallel circuit, as shown in FIG. 6B. However, these conventional equivalent circuit models do not have satisfying accuracy. As shown in FIGS. 6C and 6D, the conventional models hardly reproduce an impedance having complex frequency dependence. FIG. 6C shows a real part of the impedance, and FIG. 6D shows a capacitance component of the impedance. Calculated values of a three-element model are represented by solid lines, and calculated values of a five-element model are represented by broken lines. FIGS. 6C and 6D show large differences between the calculated values and actually-measured values given by dotted lines.
Therefore, for electronic circuits including capacitors, an estimation result of a circuit simulator does not often match with electric characteristics of an actual circuit, and this prevents electronic circuits using the circuit simulator from being designed efficiency.